Thin film thermal print heads having resistive heating elements are known in the prior art as disclosed in U.S. Pat. No. 3,609,294, THERMAL PRINTING HEAD WITH THIN FILM PRINTING ELEMENTS, issued to Richard C. Cady, Jr. et al. on Sept. 28, 1971. In this printing head, planar resistive heater elements are covered by a protective layer of material having substantial thermal conductivity and electrical resistivity. The protective layer is shaped to provide a raised area over each of the heater elements. These raised mesa-like areas are merely thicker portions of the overcoating which protects the entire structure. The mesa-like areas provide concentrations of heat in close proximity to the thermally sensitive recording material. However, since the material is selected for high thermal conductivity, conduction of heat generated by the resistive heater elements to the surface of the recording material is dissipated by transmission of some of that heat into the thinner portions of the overcoating layer. Thus the thermal efficiency of this system is necessarily low. In the present invention, the resistive elements are formed over a mesa of glass glaze which places the source of heat itself in close proximity with the thermally sensitive recording material and reduces the heat dissipated throughout a protective overcoating layer. The thermal efficiency of the present invention is therefore much higher.
Ion migration barriers are also known in the prior art as disclosed in U.S. Pat. No. 3,598,956, ION MIGRATION BARRIER, issued to Richard C. Cady, Jr. et al. on Aug. 10, 1971. The ion migration barrier disclosed therein comprises an electrically conductive shield which isolates the resistive heater elements from ions which shortened heater element life. The migration barrier is insulated from the heater elements and from the thermally sensitive recording material by layers of glass, and is most effective when electrically biased or grounded. The ion barrier of the present invention is effective without electrical biasing or grounding, and does not require an insulating layer of glass between it and the heater elements. In addition the ion barrier of the present invention enhances adhesion of the wear layer to the heater elements.
The ion barrier described above is formed during a heat treatment process which also serves to adjust the value of the resistive elements. By controlling the temperature and time of heat treatment, the resistor values may be increased to a resistance value compatible with drive electronics. Since deposits of resistive material are more uniform in thin film technology than in other fabrication processes, external trimming resistors are generally not required. However, since heat treatment effects the resistance of all heater elements uniformly, it does allow the upward adjustment of that resistance to accommodate the electronic drive circuitry.